1. Field of the Invention
This invention pertains to silicon deposition by silane pyrolysis in a fluidized bed reactor. More particularly, it pertains to an improved method for preparing high purity silicon. In an important aspect, this invention provides means for reducing the amount of silicon dust on the surface of larger silicon particles.
2. Description of the Prior Art
It is known in the art that the fluidized bed reactor offers many advantages for chemical vapor depositions. For example, the fluidized bed provides improved energy utilization and material economy; confer Wakefield, U.S. Pat. No. 4,154,870. As pointed out in that reference, continuity of operation, the large surface area of the fluidized particles, and the high exposure of solid surfaces to the gas stream, all provide economy of operation.
McHale, U.S. Pat. No. 4,292,344, pertains to production of polycrystalline silicon by decomposition of silane, or a halosilane, in a fluidized bed. It teaches that process conditions are preferably maintained so that decomposition of the silicon compound occurs in a heterogeneous manner; i.e. so that silicon is deposited on the surface of particles in the bed. However the reference points out that in conventional reactors, homogeneous decomposition of silane also takes place, resulting in the formation of fine silicon powder or dust. This material is a light, fluffy powder and is usually undesirable since it is difficult to handle.
Eversteijn, Philips Res. Repts. 26, 134-144, (1971) comprises a study of gas phase decomposition of silane in a horizontal epitaxial reactor. It was found that gas phase decomposition is a serious factor that must be taken into account. In order to avoid gas phase decomposition, the maximum silane concentration in the hydrogen admitted to the reactor was 0.12-0.14 volume percent, depending on the gas temperature. When this critical silane concentration was exceeded, gas phase decomposition occurred giving rise to silicon fines which deposited on the substrate.
The Eversteijn article is referenced in Hsu et al, J. Electrochem Soc.: Solid State Science and Technology, Vol. 131, No. 3, pp. 660-663, (March, 1984). As stated there, the success of the Siemen's process led to its universal adoption for producing semiconductor grade silicon, and the de-emphasis of fluidized bed (FB) process development. In 1975, the potential market for semiconductor grade silicon for photovoltaic use made fluidized bed (FB) production of polysilicon more attractive. Fluidized bed operation has the capabilities of high-throughput, continuous operation and low energy cost. Because silane has a low deposition temperature, and can be completely converted in a non-reversible reaction, it is attractive for use in FB operation. Additional advantages are the non-corrosive atmosphere, and ease of recycling by-product hydrogen. In conventional chemical vapor decomposition devices, there is a limit of silane concentration in hydrogen beyond which unwanted fines are homogeneously nucleated. Thus, in addition to the desired deposition, silicon dust or fines appear in the gas phase. These particles vary in size from submicron to .about.10 microns, and present mechanical problems in the operation of the reactor. They are also difficult to transport. Dust and fines are considered losses in the process. Hence, conventional reactors are operated with low silane concentrations to prevent excess fines formation. In a fluidized bed reactor, less fines are generated because (i) there is less free space available for homogeneous nucleation and (ii) the silicon particles scavenge the fines and incorporate them into the deposition growth. Consequently, the net amount of fines is less than for chemical vapor deposition apparatus, and a fluidized bed reactor can be operated at much higher silane concentrations with greater throughput. Variables which effect the amount of fines elutriated were studied. Conclusions reached were as follows:
Elutriated fines increase with increased silane concentration, increased temperature, increased gas bubble size, and increased gas velocity. The authors selected 600.degree.-800.degree. C. and a gas velocity of U/U.sub.MF =3-8 as good operating parameters.
Another article, Hsu et al, Eighteenth IEEE Photovoltaic Specialists Conference (1984) pp. 553-557, discusses additional studies on fines formation. It states that silane pyrolysis in a fluidized bed reactor can be described by a six-path process: heterogeneous deposition, homogeneous decomposition, coalescence, coagulation, scavenging, and heterogeneous growth of fines. The article indicates that fines formation can be reduced by providing at a suitable bed location, a secondary source of silane for cementation.
The cited art clearly shows that production of silicon via decomposition of silane is complicated, and that provision of improved processes is not straight forward. Nonetheless, because of continuing advances in the electronics industry and the development of new products in that field, improvements in existing technology are needed to provide high purity silicon at reduced cost. This invention, which enhances operation of fluidized bed methods by providing means to make high quality product under high productivity operating conditions, satisfies that need.